A gas turbine employs a plurality of stationary vanes, one of which is shown in FIG. 1, circumferentially arranged in rows in its turbine section. Since such vanes are exposed to the hot gas discharging from the combustion section, cooling of these vanes is of utmost importance. Typically, cooling is accomplished by flowing cooling air through cavities, such as cavities 11, 12 and 13 shown in FIG. 2, formed inside the vane airfoil. A tubular insert is disposed in each of these cavities to distribute the air within the cavity. In addition, as shown in FIG. 3, a flat plate-like member 24, referred to as an impingement plate, is attached to the outer shroud of the vane. The impingement plate has a plurality of holes formed therein to promote the formation of jets of cooling air which impinge on the outer shroud.
In order to receive the cooling air directed to the vane, the distal end of at least a portion of the inserts must form an inlet which extends beyond the impingement plate. In the past, the inlet has been created by using a single piece insert which was sufficiently long to extend beyond the impingement plate. However, it is difficult to attach such long inserts to the outer shroud because the projecting end of the insert restricts access to the portion of the insert, referred to as the cover plate, along which the insert must be welded to the shroud. Such welding access is especially restricted in the area of the rear support rail and the raised edges of the outer shroud. This lack of access for welding not only makes fabrication of the vane more costly, it often results in a poor quality weld which is prone to failure. Consequently, it would be desirable to provide an insert having an inlet which extended beyond the impingement plate but which provided sufficient access for welding of the insert to the outer shroud.
In the past, the hole in the impingement plate through which the insert extended was sealed by attaching a seal to the impingement plate which pressed against the insert --that is, the seals formed openings which had a smaller size than that of the insert so that there was an interference fit between the seal and the insert. This approach was necessary because positive sealing by welding the seals directly to both the impingement plate and the inserts was not feasible with the inserts heretofore used in the art. This is so because there was insufficient flexibility in such inserts to withstand the differential thermal expansion between the insert and the impingement plate. As a result, welding a seal to both components would cause cracking of the seals or their welds. Unfortunately, the interference fit between the seal and the insert is sometimes lost after extended operation due to wear and creep, resulting in the leakage of cooling air. Consequently, it would be desirable to provide inserts having sufficient flexibility to allow positive sealing by incorporating seals which were welded to both the impingement plate and the inserts.